1. Field of the Invention
The present invention relates to a method of manufacturing a photonic crystal and a method of manufacturing a surface-emitting laser.
2. Description of the Related Art
A surface-emitting laser is known in which a high contrast grating (hereinafter referred to as HCG) which is a kind of a two-dimensional photonic crystal or a one-dimensional photonic crystal is utilized as a reflector.
A photonic crystal has a microstructure in which the refractive index is periodically modulated with a period which is equal to or shorter than the wavelength of light. A photonic crystal formed of a semiconductor often includes pores or grooves formed in the semiconductor and uses the refractive index periodical structure of the semiconductor and air.
In order to widen the wavelength bandwidth of the reflector formed of a photonic crystal, it is effective to form an air gap so that a top portion and a bottom portion of the photonic crystal are brought into contact with air to widen the refractive index difference between the photonic crystal and layers over and under the photonic crystal.
As a method of forming an air gap (cavity) under the photonic crystal, a method is well known in which the photonic crystal is formed on a semiconductor layer (sacrifice layer) that can be removed by wet etching and then the sacrifice layer is removed to form the cavity.
However, for example, a nitride semiconductor is difficult to be wet etched, and a photonic crystal formed of a nitride semiconductor has a problem in that it is difficult to form a cavity using such a sacrifice layer.
In order to form such a cavity, Japanese Patent Application Laid-Open No. 2002-261032 proposes a method of manufacturing a nitride semiconductor substrate which is described in the following.
In this method, a protective film having openings formed therein is formed on a support substrate formed of a nitride semiconductor, a first nitride semiconductor is laterally overgrown on the protective film from exposed portions of the support substrate, the growth is stopped so that clearance is provided on the protective film between portions of the first nitride semiconductor layer, and then, the protective film is removed.
By removing the protective film, space is formed below the clearance between the portions of the laterally overgrown first nitride semiconductor.
After that, a second nitride semiconductor is grown from the upper surfaces, or the upper surfaces and the side surfaces, which are laterally overgrown portions, of the first nitride semiconductor, and then the second nitride semiconductor are connected to each other over the spaces. Thus, cavities are formed below the portions of the second nitride semiconductor connected to each other.
However, the method disclosed in Japanese Patent Application Laid-Open No. 2002-261032 cannot remove the nitride semiconductor which is grown on the openings in the protective film, that is, on the exposed portions of the surface of the support substrate, and thus, cannot form the cavities over the exposed portions.
Therefore, when cavities are formed below a photonic crystal by forming the photonic crystal on the semiconductor layer above the cavities formed using the method disclosed in Japanese Patent Application Laid-Open No. 2002-261032, there is a problem in that there are a lot of regions in which such cavities cannot be formed.
The present invention has been made in view of the above-mentioned problem, and has an object to provide a method of manufacturing a photonic crystal and a method of manufacturing a surface-emitting laser which are capable of forming a cavity below a photonic crystal when the photonic crystal formed of a semiconductor that is difficult to be wet etched is manufactured.